The present invention relates to a protection circuit for an EMI/RFI suppression system.
The invention also concerns a method for protecting an EMI/RFI suppression system.
Nonlinear electrical loads having a non-sinusoidal input current such as a steep-rising current pulse, generate harmonic distortions of the nominal frequency on the power network. Such load are, e.g., miniature fluorescent lamps equipped with an electronic ballast. These lamps consume a current pulse of only 1.5 ms duration at the peak of the 50 Hz sinusoidal mains voltage waveform. In a modern office environment, the entire environment may consist of similar low-quality loads such as computers, copiers, printers and air-conditioning systems equipped with electronic fan speed controllers. The dominant harmonic current component caused by such loads, specifically the third harmonic, is summed almost arithmetically on the neutral conductor of the power network. In practice, current peaks which nearly double the phase leg current have been measured on the neutral conductor. Since the conventional neutral conductor has no overload protection, a potential hazard is created under high-load conditions. Moreover, the third harmonic has been found to cause disturbances in data-processing equipment and communications facilities.
Conventionally, such disturbances have been overcome by connecting a bandstop filter, tuned to the third harmonic of the mains frequency, between the load grounding point and the star point of the main transformer.
This arrangement, however, creates a problem if, e.g., in a five-conductor system the protective-earth-grounded equipment housing is incorrectly connected to the neutral conductor, that is, if an improper grounding connection is made. In such a situation, since the neutral connector is essentially isolated from ground for the third harmonic due to the tuned nonstop filter, the distortion current component at this harmonic is reflected back to the point of the incorrect connection, thereby creating a hazardous situation.
Networks may be provided with earth-fault detection systems which are mandatory, e.g., in medical installations and explosion-hazardous environments. Such systems are conventionally based on either a sum current transformer or a current transformer placed on the protective earth (PE) conductor. The low-resistance circuit (water piping, conductive building structures) connected in parallel with the PE conductor make it very difficult to detect fault situation. Therefore, conventional earth-fault detection systems are frequently hampered by false fault alarms.